Microsoft Internet Explorer and Google Chrome are doing it, so why should not the Firefox web browser? Running plugins in separate processes adds much to a web browser’s stability as a crash in a plugin could, and usually would, result in a browser crash as well if everything was handled under one process. With out-of-process plugins on the other hand the only damage that is done is to the plugin’s process which gets restarted, the rest of the browser is not taken down with that plugin crash anymore.
The Mozilla developers have now released a beta version which they have called Firefox 3.6.3 Plugin 1 Lorentz. It is a beta release and therefor not available on the main site or through the Firefox automatic update checker.
The version that has been released is basically Firefox 3.6.3 with the Out-of-process plugin feature enabled.

The new feature currently supports the three popular plugins Adobe Flash, Apple Quicktime and Microsoft Silverlight. If those are installed they are running in their own process with the new release. The user will be notified about plugins crashing on the page where it happened. The page and the rest of the browser will continue to work as usual.
Firefox 3.6.3 Plugin 1 Lorentz can be downloaded from the Mozilla developer pages where it is available for all supported operating systems. Please note again that this is a beta release and should not be installed in a productive environment.
Enjoyed the article?: Then sign-up for our free newsletter or RSS feed to kick off your day with the latest technology news and tips, or share the article with your friends and contacts on Facebook or Twitter.Related Articles:
Firefox 3.6.4 Beta ReleasedWhy Are 2 Plugin-Container.Exe Processes Running?
Run Custom Firefox Plugins In Their Own Process
Multi-Process Plugins Enabled In Latest Firefox Nightlies
Check and Remove Plugins in Firefox

Waiting for official release
Nice feature, good update from Mozilla, Yes, I am also waiting for Official Release.
I installed Firefox 3.6.4pre Lorentz. So far no crashes, but some pages like Yahoo mail flash page or RipoffReport report page do not work. And menus do freeze like before. Silverlight work choppy. But flash video works fine.
So, program becomes more reliable but less functional. Typical pre release. I will probably downgrade.
You can always just install into a different dir and choose to not make it default browser if you just want to check it out and/or decide if you want to replace stable with this.
dont use wacky plugins like Qtime or Silverlight (gone with the wind LoL) is the simple solution :P